The present invention relates to a method used in a communication system and related communication device, and more particularly, to a method of handling power reduction at a transmitter and related communication device.
A modern wireline communication system, such as an asymmetric digital subscriber line (ADSL) system or a power line communication (PLC) system, usually adopts orthogonal frequency-division multiplexing (OFDM) or discrete multi-tone modulation (DMT) for transmitting and receiving signals. That is, a transmitter processes the signals by using the OFDM or the DMT before transmitting the signals to a receiver. Correspondingly, the receiver recovers the transmitted information by demodulating the received OFDM signals or the received DMT signals. In detail, when the OFDM or the DMT operates, the transmitter allocates various numbers of bits to subcarriers according to a tone map (i.e., bit loading), to maximize throughput. In general, the tone map is determined by the receiver by performing channel estimation on the channel between the transmitter and the receiver, and then transmits or indicates the tone map to the transmitter.
On the other hand, power spectral density (PSD) mask specified for the PLC system (e.g. HomePlug AV2 or G.hn) is special in that power levels at a low band (i.e., <30 MHz) and a high band (i.e., 30˜100 MHz) are greatly different. In detail, the power level at the low band is much higher than the power level at the high band. That is, when the signals are allocated to an entire band, part of the signals allocated to the low band can be transmitted with a high power level, while the other part of the signals allocated to the high band can only be transmitted with a low power level. When the receiver receives the signals at the entire band, the receiver needs to reduce the high power level of the part of the signals (i.e., the high power level at the low band), to operation region of an analog front end (AFE) of the receiver, such that the AFE can operate regularly. However, when reducing the high power level of the part of the signals, the power levels at the entire band are reduced at the same time. That is, the low power level of the other part of the signals is also reduced, such that the other part of the signals (i.e., the signals allocated to the high band) becomes weak and can not be easily recovered. Thus, reducing the power levels at the receiver is not an effective way to deal with strong signal which is generally encountered when the distance between the transmitter and receiver is short.
Therefore, the HomePlug AV2 and the G.hn both propose solutions for the power reduction, wherein the power levels are reduced at the transmitter, but not at the receiver. Notice that the power reduction is applied only to the low band portion according to the solutions proposed by the HomePlug AV2 and the G.hn. In detail, in the HomePlug AV2, reduced power levels are first determined by respective receivers, and are then transmitted to the transmitter. After the transmitter collects the reduced power levels transmitted from all the receivers, the transmitter determines a corresponding reduced power level for each receiver under a constraint that no additional hidden node is introduced. Then, the transmitter reduces a power level of a signal to the corresponding reduced power level, before transmitting the signal to the receiver. In other words, when determining the corresponding reduced power level for the receiver, the transmitter not only considers the reduced power level fed back by the receiver, but also considers the reduced power levels fed back by other receivers, to satisfy the constraint that no additional hidden node is introduced. Thus, the corresponding reduced power level is not optimal for the receiver. Besides, a large amount of time and resource required for collecting the reduced power levels from all the receivers makes this solution not efficient. On the other hand, in the G.hn, the transmitter only considers the reduced power level fed back by the receiver, when determining the corresponding reduced power level for the receiver. In this situation, additional hidden node may be introduced, since the transmitter transmits the signal with a weak power level to the receiver according to the corresponding reduced power level.
Therefore, the power reduction can not be solved completely according to the abovementioned solutions. How to solve the power reduction completely is a topic to be discussed.